Transient and steady-state drag in foam

The steady-state force exerted on a sphere moving at constant speed through an aqueous foam is studied experimentally. The dependence of the force on container size indicates that the moving sphere is surrounded by a fluidised region that extends approximately one sphere radius into the foam. The force increases with the speed of the sphere, but with a finite zero-speed intercept which is a measure of the yield stress of the foam. The steady-state force as a function of pulling speed yields a flow curve which can be described by a Herschel-Bulkley constitutive relation. The transient build-up of the force when motion starts, and its relaxation when motion stops, are also studied. While the initial build-up has a single time constant inversely proportional to the imposed shear rate, the relaxation involves three distinct processes which we relate to events on the scale of the individual bubbles making up the foam.